Why a mountain bike? Why not a road bike?
So I'm partial, I'm a roadie by trade but played on the single track in the past.
Find a reputable bike shop and ask alot of questions, test a bunch and then ask more questions. When shopping for a bike, look at the important parts such as drivetrain. You can skimp on some other parts but the things that make you go are important. I wouldn't recommend you running out and getting Dura Ace or Diore either.
I remember back in the 80's there were several really good pros that were brought to the bike for knee rehab. My doc told me to stop running and ride more.
I'd try and find something used as well from Craigslist and the like.
Trek is good! </disassociated ramblings>